When a workpiece having a plate-like shape is cut and a round hole is perforated by a laser machining device, piercing is carried out firstly on the periphery of a circle or the inside of a circle. Piercing means an operation for making a hole penetrating the plate-like workpiece at a start point of cutting. Laser beam is irradiated along a circle (circle to be cut) of the object starting from the hole perforated by the piercing operation.
In the case of a thin steel plate, a hole can be perforated instantaneously by irradiation of laser beam. Therefore, in machining a thin steel plate, piercing can be made by ON/OFF of laser beam while a machining head is moving. Further, a path leading to the circle to be cut out can be reached by smoothly moving a machine head from a position of pierced hole, and the circle can be cut out without stopping the machine head by providing some overlapping cutting path beginning from the end point of the cutting path.
FIG. 8 illustrates a cutting path of a conventional circular hole cutting operation. According to this example, the position of a machining head before starting the cutting operation for a round hole 70 is set to point P.sub.20, and a start point for machining a shape to be machined next to the round hole 70 is set to point P.sub.26. The position of piercing is set to P.sub.22.
In respect of the movement path in this case, straight line movement is made for a segment from point P.sub.20 to point P.sub.21, a circular arc movement in the counterclockwise direction is made for a segment from point P.sub.21 to point P.sub.22. A circular movement in the clockwise direction is made for a segment from point P.sub.22 to point P.sub.23. Output of a laser beam is started at point P.sub.22. A circular movement in the clockwise direction is conducted from point P.sub.23 along the round hole (circle to be cut) 70 until the movement makes a round and returns to point P.sub.23. A circular movement for an overlapped path is made from point P.sub.23 to point P.sub.24, thereby forming the round hole 70.
A circular movement is conducted from point P.sub.24 to point P.sub.25. Output of the laser beam is stopped at point P.sub.24. Then, a linear movement is made therefrom to the next machining position of point P.sub.26.
All of the movement paths from point P.sub.20 to point P.sub.26 constitute a smooth path. Therefore, the desired round hole can be formed by cutting with the laser beam turned on and off while the machining head is moving continuously without stopping in the middle of machining. As a result, a marked reduction in machining time period is realized.
However, the acceleration in the circular arc movement is increased in proportion to a square of the velocity, and so, when the machining speed is accelerated, the change in the acceleration cannot be disregarded. There exists a point where the acceleration largely varies even with the above-described smooth path. Such change in the acceleration causes the path of the machining head to vibrate and deteriorate machining accuracy.
For example, with regard to FIG. 8, the direction of the movement on the circular arc is reversed at point P.sub.22. The acceleration in the case of the circular movement is directed toward the center of the circular arc, and, therefore, the direction of the acceleration changes by 180.degree. at point P.sub.22. Such a large change in the acceleration gives an impact on a moving shaft. The impact acting on the movement shaft gives adverse effect on a servo control system, causing the path of the machining head to be vibrated. The vibration of the path of the machining head will not cease immediately, and, therefore, the vibration occurring at point P.sub.22 gives adverse effect on the path on the outer periphery of the round hole to be formed by cutting. As a result, the machining accuracy is adversely affected.